I'd just get them hitting, catching, throwing balls wherever they are just to help their development. Keep a bat and ball in your car so if you go out for the day somewhere they can always play. With your 8 year old I would try and teach him some basic shots. Drive, pull (of a slow bowler), forward defence. Most shots then snowball on from these. With your 5 year old, just keep him hitting balls (probably off a tee? Not sure on his ability), as long as he has the grip, stance and set up right then it ok, you can bring in specific shots when he a bit older. Bowling you could try with both of them, just do it from the one step position. As long as your 5 year old can grasp the way the arm comes over and its straight then the rest can come later.
As long as they don't turn around and say. "Dad I'm bored and want to stop" then you are doing ok!